The present invention relates to coated metal sheet which can be bonded with an adhesive, and is useful as the casing of electric home appliances or business machines, building material, furniture or the like.
Coated steel sheets have the undercoat and top coat paint films composed of thermosetting film-forming resin on the surface of the steel sheets which have been subjected to chemical converion treatment such as phosphating or chromating. Such coated steel sheets, especially steel sheets precoated in factories, have been used as structural parts or members for electric home appliances, business machines, building or furniture, since they have beautiful external appearance without the necessity of paint application at a user's side.
However, when the coated sheet member is fixed by welding or calking, its external appearance is significantly deteriorated due to the damage of the paint film at the welded or calked part. The paint film is deformed to a state easy to separate from the steel substrate by the welding or calking, too. Since the defects formed in the paint film act as the starting points to promote corrosion, the external appearance becomes inferior due to the trace of rust flow.
The deterioration of paint film can be avoided by bonding coated steel sheet members with an adhesive instead of welding or calking. For instance, coated steel sheets are bonded with a hotmelt-type adhesive in the presence of an intermediate liquid film having sufficient wettability, as disclosed in Japanese Patent Application Laid-Open 3,134083. Various kinds of bondable coated steel sheets are proposed in Japanese Patent Applications Laid-Open 62-225341 and 64-85753.
Recently, the demand for coated steel sheets having good workability has become stronger. In response to the demand, the composition of polyester resin with melamine resin has been used as the paints which are applied to form undercoat and topcoat films on a steel substrate. However, the combination of polyester resin with melamine does not sufficiently satisfy the requirements for the purpose. Even when the coated steel sheets are bonded with a structural adhesive such as two-pack liquid acryl resin, initial adhesiveness is poor, and adhesion durability is likely reduced with the lapse of usage over a long period. Especially, when the coated steel sheet after being bonded is exposed to a wet atmosphere, the adhesion durability becomes poor in a short time. Consequently, the structural body assembled with an adhesive is not suitable for long-term use, due to poor durability.